1. Field of the Invention
The present invention relates to an injection molding screw for metals which is used for melting and injection molding non-ferrous metals including zinc, magnesium or the alloys thereof having a low melting point.
2. Detailed Description of the Related Art
The molding of non-ferrous metals including lead, zinc, magnesium and tin having a low melting point or the alloys thereof used to be conducted by a die-casting. By employing injection molding methods similar to that for plastic materials, the molding process is being simplified and the precision of the molding is being improved.
Notwithstanding the above, a difference in melting process between plastics and metals causes some problems.
FIGS. 3 and 4 show an apparatus to be used for injection molding. The apparatus has a nozzle 2 on the end, and a heating cylinder 1 with a feeding opening 3 on the rear thereof. The heating cylinder 1 contains a screw 7 for injection so as to be rotational and movable thereinside. The screw 7 is provided with a conical tip end 4 and a screw flight 6 around an axis 5 thereof.
In the injection molding of a plastic material, the plastic material is melted by a shear heat generated by the rotation of screw plasticization. Once injecting the metered melting plastic material by the fore end portion of the heating cylinder through forwarding the screw, during the plastication by the rotation of the screw, the plastics will be metered through the backing of the screw by the pressure from the stored materials.
Therefore, for the injection screw for plastics, the end 6a of the screw flight 6 is needed to be positioned below rear edge 3a of the feeding opening 3 at the foremost position of the screw, as shown in FIG. 3. In this configuration, a screw groove 6b is needed to be always formed facing the feeding opening 3.
On the other hand, the metals are melted with external heat. The melted metals in liquid phase have low viscosity as differed from plastics. Therefore, the metals are transferred to the fore end of the heating cylinder by the rotation of the screw. However, since they cannot generate enough pressure to move the screw backward, the metering is performed by backing the screw mechanically and forcedly. The materials are transferred into the front of the heating cylinder by the rotation of the screw at the rearmost position of the screw.
In the injection molding of the metals as stated above, when the screw for injection molding of plastics is employed, as shown in FIG. 4, at the rearmost position of the screw, the rear end 6a of the screw flight 6 will be positioned behind the feeding opening 3. At the same time, the metals left in the screw groove 6b will be transferred and stay behind the feeding opening 3.
The materials left therein will be forwarded by the rotation of the screw. Since there is the feeding opening 3 before the left materials when the screw is in the rearmost position, the metal materials are additionally fed into the screw groove below the feeding opening 3. Therefore, the left materials have a tendency to stay therein, which impairs the screw rotation or sliding.
When the screw with the screw groove 6b always facing the feeding opening 3 is employed, the materials are ready for being transferred into the heating cylinder by the screw rotation regardless of screw position. Hence, when the screw is rotated for a purpose besides the plasticization, the screw groove 6b is fed and may be congested with the materials from the feeding opening 3. To prevent the congestion, feeding the materials into the feeding opening 3 is temporarily stopped, which causes inconvenience.
The present invention is aimed to solve the above-mentioned problems. An object of the present invention is to provide an injection screw for metals which allows to prevent the materials from being left, and reduce the friction of the screw rotation and sliding only by limiting the end position of the screw flight to a certain position according to the position of the feeding opening in the heating cylinder.
In order to achieve the above-mentioned object, the present invention provides an injection molding screw for metals, which is rotationally and movably provided in a heating cylinder having a nozzle at a tip end thereof, for transferring granular metals fed thereinside from a feeding opening at the rear of the heating cylinder while melting the metals, and for injecting the melted metals metered in the fore end of the heating cylinder from the nozzle by the forward movement of the screw, wherein a screw flight is formed around an axial portion of the screw, of which rear end is positioned below a rear edge of the feeding opening at the rearmost position of the screw in the heating cylinder, and wherein at the foremost position of the screw, the rear end thereof is positioned in front of the feeding opening in order to close the feeding opening by a rear portion of the axial portion of the screw.
With the screw as mentioned above, as the feeding opening is closed along with the forward movement of the screw by the axial portion, the congestion of the metals in the screw grooves at the end of the screw by automatically controlling the feeding of the materials when the injection starts. Thereby, the friction of the screw rotation and sliding decreases. The metals will be melted and injected steadily and the quality of the molded products will be improved.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.